The size of a semiconductor nanoparticle generally dictates the electronic properties of the material, the band gap energy being inversely proportional to the size of the semiconductor nanoparticle as a consequence of quantum confinement effects. In addition, the large surface-area-to-volume ratio of the nanoparticle may have a profound impact upon the physical and chemical properties of the nanoparticle.
Two fundamental factors, both related to the size of the individual semiconductor nanoparticle, are primarily responsible for their unique properties. The first is the large surface-to-volume ratio: as a particle becomes smaller, the ratio of the number of surface atoms to those in the interior increases. This leads to the surface properties playing an important role in the overall properties of the material. The second factor is that, with many materials including semiconductor nanoparticles, the electronic properties of the material change with size. Moreover, because of quantum confinement effects, the band gap typically gradually becomes larger as the size of the particle decreases. This effect is a consequence of the confinement of an ‘electron in a box’ giving rise to discrete energy levels similar to those observed in atoms and molecules, rather than a continuous band as observed in the corresponding bulk semiconductor material. Thus, for a semiconductor nanoparticle, because of the physical parameters, the “electron and hole,” produced by the absorption of electromagnetic radiation, a photon, with energy greater then the first excitonic transition, are closer together than in a corresponding macrocrystalline material. Moreover the Coulombic interaction cannot be neglected. This may lead to a narrow bandwidth emission that is dependent upon the particle size and composition of the nanoparticle material. Thus, quantum dots generally have higher kinetic energy than the corresponding macrocrystalline material and consequently the first excitonic transition (band gap) increases in energy with decreasing particle diameter.
Core semiconductor nanoparticles that consist of a single semiconductor material along with an outer organic passivating layer tend to have relatively low quantum efficiencies due to electron-hole recombination occurring at defects and dangling bonds situated on the nanoparticle surface that can lead to non-radiative electron-hole recombinations.
One method to eliminate defects and dangling bonds on the inorganic surface of the quantum dot is to grow a second inorganic material, having a wider band-gap and small lattice mismatch to that of the core material epitaxially on the surface of the core particle, to produce a “core-shell” particle. Core-shell particles separate any carriers confined in the core from surface states that would otherwise act as non-radiative recombination centres. One example is ZnS grown on the surface of CdSe cores.
Another approach is to prepare a core-multi shell structure where the “electron-hole” pair is completely confined to a single shell layer consisting of a few monolayers of a specific material such as a quantum dot-quantum well structure. Here, the core is of a wide band gap material, followed by a thin shell of narrower band gap material, and capped with a further wide band gap layer, such as CdS/HgS/CdS grown using substitution of Hg for Cd on the surface of the core nanocrystal to deposit just a few monolayers of HgS which is then overgrown by monolayers of CdS. The resulting structures exhibited clear confinement of photo-excited carriers in the HgS layer.
To add further stability to quantum dots and help to confine the electron-hole pair, one of the most common approaches is to epitaxially grow a compositionally graded alloy layer on the core. This can help to alleviate strain that could otherwise led to defects. Moreover, for a CdSe core, in order to improve structural stability and quantum yield, rather than growing a shell of ZnS directly on the core, a graded alloy layer of Cd1-xZnxSe1-ySy may be used. This has been found to enhance the photoluminescence emission of the quantum dots.
Doping quantum dots with atomic impurities is an efficient way also of manipulating the emission and absorption properties of the nanoparticle. Procedures for doping of wide band gap materials such as zinc selenide and zinc sulphide with manganese and copper (ZnSe:Mn or ZnS:Cu) have been developed. Doping with different luminescence activators in a semiconducting nanocrystal can tune the photoluminescence and electroluminescence at energies even lower than the band gap of the bulk material, whereas the quantum size effect can tune the excitation energy with the size of the nanocrystals without causing a significant change in the energy of the activator related emission.
The coordination about the final inorganic surface atoms in any core, core-shell or core-multi shell, doped or graded nanoparticle is incomplete, with highly reactive, non-fully coordinated atoms “dangling bonds” on the surface of the particle, which can lead to particle agglomeration. This problem may be overcome by passivating (also referred to as “capping”) the “bare” surface atoms with protecting organic groups.
An outermost layer of organic material or sheath material (referred to as a “capping agent”) helps to inhibit particle aggregation and protects the nanoparticles from their surrounding electronic and chemical environment. A schematic illustration of such a nanoparticle is provided in FIG. 1. In many cases, the capping agent is the solvent in which the nanoparticle preparation is undertaken, and includes a Lewis base compound or a Lewis base compound diluted in an inert solvent, such as a hydrocarbon. The lone pair of electrons on the Lewis base capping agent are capable of a donor-type coordination to the surface of the nanoparticles. Suitable Lewis base compounds include mono- or mulit-dentate ligands, such as phosphines (trioctylphosphine, triphenolphosphine, t-butylphosphine), phosphine oxides (trioctylphosphine oxide), alkyl phosphonic acids, alkyl-amines (hexadecylamine, octylamine), aryl-amines, pyridines, long chain fatty acids and thiophenes, but is not restricted to these materials.
The widespread exploitation of quantum dot nanoparticles has been restricted by their physical/chemical instability and incompatibility with many applications. Consequently, a series of surface modification procedures has been employed to render the quantum dots more stable and compatible with a desired application. This has been attempted mainly by making the capping agent bi- or multi functional or by overcoating the capping layer with an additional organic layer that has functional groups that can be used for further chemical linkage.
The most widely used quantum dot surface modification procedure is known as ‘ligand exchange’. The ligand molecules that inadvertently coordinate to the surface of the quantum dot during the core synthesis and shelling procedure are subsequently exchanged with a ligand compound that introduces a desired property or functional group. Inherently, this ligand exchange strategy reduces the quantum yield of the quantum dots considerably. This process is illustrated schematically in FIG. 2.
An alternative surface modification strategy interchelates discrete molecules or polymer with the ligand molecules that are already coordinated to the surface of the quantum dot during the shelling procedure. These post synthesis interchelation strategies often preserve the quantum yield but may result in quantum dots of substantially larger size. This process is illustrated schematically in FIG. 3.
Current ligand exchange and interchelation procedures may render the quantum dot nanoparticles more compatible with their desired application but typically results in lower quantum yield due to damage to the inorganic surface of the quantum dots and/or an increase in the size of the final nanoparticles.